ORIGINS OF MASSIVE-TYPE SANDSTONES IN BRAIDED RIVER SYSTEMS

This study details largely ignored massive-type, predominantly structu reless sandstones preserved within braided fluvial successions of Carb oniferous to Triassic age. Architectural element analysis reveals that these sediments were deposited within sand-dominated perennial system s of low braiding index. Cross-stratified braid bar deposits are inter bedded with, and laterally equivalent to geometrically distinct, large ly structureless massive-type sandbodies identified as two separate ar chitectural elements: channel-like (SMC) and sheet-like (SMS). Sub-div isions within these broad categories define six geometric units which are texturally distinct from each other and from the structured sedime nts of the same lithological unit. Since massive-type sandstone elemen ts have many features in common with the deposits of highly concentrat ed, laminar sediment/water flows, they are interpreted in terms of sim ilar depositional processes. SMC elements form elongate channel-like f eatures which trend both at high angles to, and parallel with, the pal aeoflow of host fluvial channels. The lower bounding surfaces of SMC e lements may be either erosive or non-erosive, and describe symmetrical cross-sections with margins dipping < 50 degrees. Concentric laminae are preserved parallel to the scour margins which grade into a structu reless sandstone fill. Diffuse laminae and water escape structures are commonly preserved in the upper portion of these elements, which are interpreted as the deposits of sandy debris flows related to fluvial b ank and/or bar collapse. SMS elements form sandsheets up to 8 m in thi ckness which may be traced > 250 m parallel and transverse to the fluv ial palaeoflow direction established from cross-stratified sandstones of adjacent architectural elements. The basal surface of SMS elements may either be undulose (where the sandbodies are termed SMSU) or erosi onal (where they are termed SMSE). Internally SMSU elements preserve p arallel laminae marginal to basal scours, diffuse sweeping laminae, is olated cross-sets and water escape structures. The SMSU sediments are compositionally identical to the structured sandstones with which they are associated and are interpreted as the result of deposition from h ighly concentrated stream flows developed through flooding and entrain ment of sandy floodplain material. Along the basal surface of SMSE ele ments scours in excess of 4.5 m are preserved which are elongate both parallel and oblique to fluvial flow. These are commonly lined with a mudclast breccia and/or massive sandstone with crude laminae developed parallel with the margin. In the upper portion of SMSE elements diffu se dune structures and sweeping laminae are preserved. SMSE units are texturally and compositionally immature with respect to the cross-stra tified sandstones with which they are associated. Thus, they are belie ved to be of first cycle origin and are interpreted as the deposits of highly concentrated flows possibly generated by glacial outburst floo ds or jokulhaups. (C) 1998 Elsevier Science B.V. All rights reserved.